This study will examine the endogenous melatonin levels, temperature and activity patterns in children with Angelman syndrome, and the acute behavioral, temperature, and hormonal changes that occur when these children are exposed to physiological doses of the hormone an hour before bedtime. Levels of motor performance, skin and core temperature will be continuously assessed. Blood samples will be taken via an indwelling venous catheter and saliva samples will be collected for determination of melatonin levels. Core and skin temperature will be measured to determine whether acute changes in melatonin concentration effect the circadian temperature pattern, and whether there is a correlation between melatonin's effect on sleep and the body temperature. Recently physiological doses of melatonin, the main hormone of the pineal gland, have been shown to induce sleepiness in healthy young humans, to decrease latency to sleep onset and latency to stage 2 sleep, as well as to decrease latency to sleep during night awakenings.